This invention relates to a magnetic core and a process for producing the same, more particularly to a magnetic core which is high in magnetic flux density, small in increase of core loss in high frequency region and excellent in frequency characteristics of magnetic permeability, and a process for producing the same.
As a method for production of magnetic cores well known in the art, there is the method in which magnetic powder and a resin are mixed and compression molded, as disclosed in Japanese Patent Publications Nos. 22514/1972 and 22515/1972 and U.S. Pat. Nos. 4,502,982 and 4,543,208. According to such a method, magnetic cores with excellent characteristics, having high saturated magnetic flux density and high magnetic permeability, can be obtained.
On the other hand, uses of such magnetic cores may include, for example, various reactors for power amplifiers, magnetic amplifiers, etc. or uses for transformers. In such uses, a current of considerably high frequency may sometimes flow. For example, in the case of a reactor to be used in a switching circuit, a current of switching frequency which is about some 10 Hz to 200 KHz, or further a current of high frequency of, for example, 500 KHz or higher may sometimes flow. The core loss in the alternating current magnetic characteristics is indicated by the sum of the hysteresis loss and the eddy current loss, and the eddy current loss which is increased in proportion to the second power of frequency becomes predominant as the frequency becomes higher. Accordingly, when use at high frequency is considered, it becomes necessary to suppress the eddy current loss, and the core loss can be made smaller by making the eddy current loss smaller. Also, generally speaking, when the rate of core loss increased is great, magnetic permeability tends to be lowered with increase of frequency. Even so, lowering in the magnetic permeability should desirably be small also in the high frequency region.
Here, according to the method as disclosed in Japanese Patent Publication No. 22514/1972 as mentioned above, while the effects such as high magnetic permeability, high saturated magnetic flux density, etc. can be obtained, low core loss and high magnetic permeability in high frequency regions exceeding 10 KHz could be obtained with difficulty. This may be considered to be due to the fact that the magnetic core was produced by the above method by mixing magnetic powder with a resin, followed by compression molding, whereby the thickness of the polymer layer covering over the surface of magnetic powder is liable to become non-uniform, and the insulation between magnetic particles tends to be unsatisfactory. For improvement of insulation, one may consider to increase the amount of the resin to be mixed, but this approach results in reduced amount of magnetic powder in the magnetic core obtained and, also, the magnetic resistance between magnetic particles will become unnecessarily greater, whereby magnetic characteristics such as magnetic flux density will be lowered.
Thus, in production of magnetic cores, the crucial issue is how to effect efficient and good insulation between magnetic particles.